Tabular grains are crystals possessing two major faces that are substantially parallel in which the average diameter of said faces is at least three times (and often many more times) the distance separating them.
Silver bromide photographic emulsions containing a high proportion of crystals having a tabular or plate-like shape can readily be prepared according to Berry et al, Photographic Science and Engineering, 1961, Volume 4, Pages 332-333 in which a defined high excess of bromide ion, the concentration being specified as pBr 0.77 is present in the emulsification medium during the growth of the crystals, which is conducted by balanced double jet addition. This defines the basic conditions for satisfactory growth of this type of crystals. In common with other types of emulsion it is also useful to apply well know growth methods such as the use of a low initial rate of addition for the formation of the first small nuclei, increasing the rate of addition continuously or stepwise to a higher rate, as the crystals grow in diameter.
The tabular crystals in emulsions made by the above method, or modifications thereof having different addition rate procedures, additions of iodide, or slightly modified bromide excess conditions not exceeding pBr 1.1, have large diameters, often in excess of 2 microns, and are also thin, typically 0.1 microns or less between the major faces, so as to have typical average ratios of diameter/thickness of 20:1 to 30:1. The use of such emulsions in colour negative and x-ray materials is disclosed in U.S. Pat. Nos. 4,433,048, 4,435,449, 4,439,520, and other related patents.
Whilst tabular grains in general can be expected to have advantages of good developability and increased useful adsorption of sensitising dye per weight of silver due to their high surface area-to-volume ratio, those of very high diameter/thickness ratio also have certain disadvantages. One of these is stress marking and associated problems due to their fragility and ease of physical distortion under mechanical strain. The grain size distribution curve of the emulsion tends to have a tail indicating the presence of larger grain sizes, so that a typical emulsion having a mean grain diameter of 1 to 2 microns can contain a significant proportion of grains 4 or more microns in diameter. These, and the thin needles which are usually also present, are more susceptible to physical damage and fog formation. Satisfactory chemical sensitisation and stabilisation are also more difficult with thin tabular grains than with conventional grains, so that post-coating instability can be a serious problem. The silver image developed from thin tabular grains has a very noticeable reddish-brown hue, which is a serious disadvantage for medical x-ray films, in which the hue is displayed prominently in the diagnostically important low-to-middle density regions, and is unacceptable to radiologists.
Thick tabular grains, e.g. having diameter/thickness ratios of below 12:1 can be expected to overcome most of these problems. It is known to make emulsions in which thick tabular grains are present by using a pBr substantially above or below pBr 0.77 or, as is very common in traditional emulsions, by adding silver throughout a range of pBr in this region, starting with a high halide concentration. An example of such an emulsion is disclosed in U.S. Pat. Nos. 4,210,450 and 4,425,426. Also, the presence throughout crystal growth of substantial amounts of non-halide AgX solvents, such as ammonia or various sulphur compounds, results in the presence of thick grains of tabular appearance, as in the traditional "ammoniacal" emulsions. A further method is to commence emulsification with a core addition of iodide, or to use non-tabular seed crystals of silver iodide or iodobromide, as disclosed in U.S. Pat. No. 4,184,878.
These methods do not enable the final thickness of the crystals to be controlled at will during growth, and many are unsatisfactory in respect of giving emulsions having crystals of predominantly very low diameter/thickness ratios, in some cases with mixtures of crystals of different thicknesses and morphology.
European Patent Application No. 0263508 discloses a process for the preparation of a photographic emulsion containing tabular silver halide grains, which exhibit high speed upon sensitisation, having a thickness of about 0.05 to 0.5 .mu.m, average grain volume of about 0.05 to 1.0 .mu.m.sup.3, and a mean aspect ratio of greater than 2:1 comprising:
a) adding silver nitrate to a vessel containing a dispersing medium/bromide mixture wherein the initial bromide ion concentration is 0.08 to 0.25 normal whereby tabular seed grains are formed,
b) adding an ammoniacal base solution to achieve 0.002 to 0.2 normal of the base after at least 2% of the total silver nitrate has been added to the vessel, and,
c) adding silver nitrate and halide taken from the group consisting of Br.sup.- and BrI.sup.- by balanced double jet procedure whereby tabular grains are formed.
U.S. Pat. No. 4,722,886 discloses a process for the preparation of a photographic emulsion containing tabular silver halide grains having a narrow size distribution comprising:
a) adding silver nitrate to a vessel containing a dispersing medium/bromide mixture wherein the initial bromide ion concentration is 0.08 to 0.25 normal, whereby tabular seed grains are formed,
b) adding a basic silver halide solvent solution to achieve 0.02 to 0.2 normal of the solvent after at least 2% by weight of the total silver nitrate has been added to said vessel,
c) stopping silver nitrate addition for a time period of 0.5 to 60 minutes to permit the tabular seed grains to ripen wherein the bromide ion concentration is in the range of 0.005 to 0.05 normal,
d) neutralizing at least some of the solvent that is present, and,
e) adding silver nitrate and halide taken from the group consisting of Br.sup.- and BrI.sup.- by balanced double jet addition whereby the tabular grains of narrow size distribution are formed.
The specific Examples of the latter two processes add the ammoniacal base solution before 10% by weight of the total of silver nitrate has been added. In order to achieve narrow size distribution the ammoniacal base solution is added and the initial silver nitrate addition halted for a time period of from 1 to 60 minutes at a bromide ion concentration in the range 0.005 to 0.05N, thereafter at least some of the ammoniacal base is neutralised.
It has now been found that if a substantial part of grain growth is completed before ammonia is added the thickness of the crystals can be controlled at will, independently of the diameter, and a narrow grain size distribution may be obtained.